FIG. 1 illustrates a typical arrangement of a virtual microscope used for image scanning as is known in the prior art. The arrangement comprises an imaging lens 1 which focuses light originating from a slide 6 onto a line scan detector 2. The imaging lens and the line detector together make up an imaging system. As the detector 2 is a line scan detector, the image area 7 is a line. In order to produce an extended image over a larger area of the slide 6, the slide is moved relative to the imaging lens and line scan detector, as indicated by arrow 8. In this sense the slide is “scanned” by the line scan detector.
The line scan detector is typically used to image a sample prepared upon the slide. The sample may be a biological specimen for example. Typically, the sample to be imaged will have an inhomogeneous surface topography with a focus variation greater than the depth of field of the imaging system. Typically, a single scan of the slide will be approximately 1 mm wide and between 2 mm and 60 mm long. Over the scale of 1 mm, the focus of the sample very rarely exceeds the depth of focus of the imaging system (typically approximately 1 μm). However, over larger distances such as 20 mm, the change of focus of the sample can exceed the depth of field of the imaging system. There is therefore the problem that the output image produced by a line scan detector while scanning a sample is likely to have areas which are in focus and areas which are out of focus, due to changes in the surface topology of the sample. This is unacceptable, especially in cases where accurate analysis of the sample is required.
There have been various attempts made to overcome this problem. For example, U.S. Pat. No. 7,518,652 discloses the use of a focus map wherein the adjustment of focus of the imaging system during the scan is predetermined. However, this requires the whole sample to be analysed before the scan can commence which is very time consuming, or only particular points on the sample are taken and therefore areas between the points are unlikely to have good focus.
U.S. Pat. No. 7,485,834 discloses temporarily changing the focus of the imaging lens during scanning of the sample to see if there is a better focus position. However, as the scan speed of the sample increases, this means that there is less time to move the imaging lens in search of a better focus position. This means that either the scanning speed has to be kept below a certain speed, or the change of position of the imaging lens has to occur over more imaging lines, which is more difficult to interpolate the image across. Both of these scenarios are undesirable.
U.S. Pat. No. 7,330,574 discloses a 2D imaging detector which is tilted in the scanning direction such that the best-focus plane of the imaging system intercepts the surface of the sample during the scan. The sample is moved one, or a small number of frames for each frame, thereby building up a 3D scan of the sample which can be used for focus calculation. This is done before scanning, as if the process were to be performed during the scan, the data rate required by the 2D scanner would be much greater.
There is therefore a requirement to improve the focussing of a sample during an imaging scan, such that the sample can be scanned quickly, and in focus.